So A1 is suppose to have the best network.
They have reasonable prepaid plans. But it sounds like prepaid doesn't include LTE?
This page shows plans that include LTE but I don't think these are prepaid:
http://www.a1.net/internet/mobile-internet-tarife
That is, a tourist can't buy these with credit cards. I think like Free in France, you have to have a local bank account to pay for these plans.
Anyone know otherwise, try to get LTE data in Austria?